A fuel cell utilizes the energy produced by a chemical reaction to supply a voltage that can be used to power other devices. This direct conversion process increases the efficiency of power generation by removing mechanical steps required by, for example, traditional turbine plants. Additionally, the combination of higher efficiency and electrochemical processes results in an environmentally favorable product. Fuel cells generate significantly less NOx and negligible quantities of carbon dioxide compared to internal combustion engines. A fuel cell powered vehicle, for example, may generate one ten-thousandth the quantity of NOx and non-detectable quantities of CO.
The solid oxide fuel cell (“SOFC”)possesses three basic parts: An anode that produces electrons, a cathode that consumes electrons, and an electrolyte that conducts ions but prevents electrons from passing. Using hydrogen gas as the fuel passed to the anode and oxygen gas as the oxidant passed to the cathode, a current of oxygen anion charge carriers is produced according to the following reactions:    at the anode: 2H2+2O2−→2H2O+4e−    at the cathode: O2+4e−→2O2−    for the cell: 2H2+O2→2H2O+heat
A conventional SOFC utilizes a ZrO2 based ceramic for an electrolyte. The anode is conventionally fabricated from a NiO—ZrO2 composite. Lanthanum manganate (LaMnO3) based materials may be used for the cathode material.
During operation, internal losses may be experienced within the SOFC that reduce the efficiency and potential. These losses may be minimized in the electrolyte simply by making it thinner. However, the act of reducing the thickness of the electrolyte may require either the anode or the cathode to be the load bearing structure. The cathode may be the more stable of the two electrodes. The anode may experience dimensional changes during the reduction of the NiO material of ˜40 vol %.
A single cell may be thermodynamically limited by the nature of the overall reaction and the concentration of oxygen on either side of the electrolyte. A single cell may produce about one volt depending on the type of gas being oxidized. Cells may be combined in parallel (to increase current) and series (to increase voltage) to create stacks. The solid-state nature of a SOFC allows easier fabrication of stacks compared to other systems, such as those containing liquid electrolytes.
In order to supply oxygen efficiently to the electrode/electrolyte interface, the electrodes may be constructed to be highly porous, however, this porosity may reduce the mechanical tolerance of the structure. Therefore, SOFCs may be constructed in a variety of geometries. A planar cell, such as a flat plate construction may have a self supporting anode, cathode, or electrolyte sheet onto which the other components are mounted.
U.S. Pat. No. 5,217,822 issued to Yoshida describes a SOFC including an anode made of porous sintered zirconia/magnesia/nickel on which a zirconia/yttria electrolyte element is superimposed. SOFCs with this type of geometry conventionally utilize a thick support electrode to support a thin electrolyte layer. For example, the Yoshida patent describes an anode plate on the order of 4 mm thick, which suffers reduced strength when sintered, and onto which an electrolyte layer on the order of 44 micrometers thick is ultimately applied. Mechanical tests have shown that an electrode with a thickness at least greater than 100 micrometers is necessary to support an electrolyte with a thickness at most less than 20 micrometers.
Another conventional SOFC construction is the tubular geometry, which is formed by fabricating the cathode, electrolyte, and anode in concentric cylindrical layers. U.S. Pat. No. 5,741,605, issued to Gillett et al., describes a fuel stack chamber configuration, where many fuel cells may be commonly supported and connected for use in large scale power generating devices. These tubular SOFCs include self supporting electrodes on the order of 1 to 3 mm thick, between which an electrolyte on the order of 0.001 to 0.1 mm thick is placed.